Power capactor and use and method related thereto

ABSTRACT

The present invention relates to a power capacitor for high voltage and comprises at least one capacitor element ( 2   a - 2   d ) enclosed in a container ( 1 ). A space is formed between each capacitor element ( 2   a - 2   d ) and the container ( 1 ). The object of the invention is to be able to handle problems concerning oil leakage in such a capacitor. According to the invention said space is filled up by a dielectric fluid ( 10 ) comprising a gelling component. Because a gel is used instead of oil, the risk of oil leakage is eliminated. The invention also relates to use of a gel in such a capacitor and to a method for manufacture of the invented capacitor.

TECHNICAL FIELD

[0001] The present invention relates to a power capacitor of the typedescribed in the preamble to claim 1. The power capacitor in accordancewith the invention is primarily intended for a rated voltage exceeding 1kV, e.g. 5 kV, preferably at least 10 kV.

[0002] Power capacitors are important components in systems for thetransmission and distribution of electric power. Power capacitorinstallations are used primarily to increase the power-transmissioncapability through parallel and series compensation for voltagestabilisation by means of static var-systems and as filters for theelimination of harmonics.

[0003] Second and third aspects of the invention relate to use of thetype described in claim 21, and to a method of the type described inclaim 23.

[0004] Capacitors have a phase angle close to 90°, and thereforegenerate reactive power. By connecting capacitors in the vicinity of thecomponents that consume reactive power, the desired reactive power canbe generated there. Cables can thus be utilised to the full fortransmitting active power. The consumption of reactive power in a loadmay vary and it is desirable to constantly generate a quantity ofreactive power corresponding to the consumption. For this purpose, aplurality of capacitors are connected via series and/or parallelconnection in a capacitor bank. The number of capacitors required tocorrespond to the consumed reactive can be connected in. Compensatingfor consumed power by utilising capacitors in the manner described aboveis known as phase compensation. For this purpose a capacitor bank in theform of a shunt battery is arranged in the vicinity of the componentsconsuming reactive power. Such a shunt battery consists of a pluralityof capacitors connected together. Each capacitor comprises a pluralityof capacitor elements. The structure of such a conventional capacitor isdescribed below.

[0005] A shunt battery usually comprises a number of chains of aplurality of capacitors connected in series. The number of chains isdetermined by the number of phases, usually three. The first capacitorin a chain is thus connected to a cable for transmitting electric powerto the consuming component. The cable for transmitting is arranged acertain distance from the ground or from points in the surroundings withearth potential. This distance is dependent on the voltage of the cable.The capacitors are then connected in series from the first capacitor,which is connected to the cable, and downwards. A second capacitorarranged at the opposite end of the chain of series-connected capacitorsis connected to earth potential or to a point in the electrical systemhaving zero potential (e.g. non-earthed 3-phase system). The number ofcapacitors and their design are determined so that the permissiblevoltage (rated voltage) over the series-connected capacitors correspondsto the voltage of the cable. A plurality of capacitors are thereforeseries-connected and arranged in stands or on platforms insulated fromearth potential. Such a capacitor bank thus includes a plurality ofdifferent components and requires relatively large quantities ofmaterial. It also requires a relative robust construction so that thestand/platform can withstand the effects of wind, earthquakes, etc.Considerable work is thus required to construct such a capacitor bank.This problem is particularly noticeable when the capacitor bank consistsof a large number of capacitors. The capacitor bank also takes up arelatively large area on the ground.

[0006] Long cables for alternating voltage are inductive and consumereactive power. Capacitor banks for series-compensation are thereforearranged with regular spacing along such a cable in order to generatethe necessary reactive power. A plurality of capacitors is connected inseries to compensate the inductive voltage drop. In a capacitor bank forseries-compensation, as opposed to a shunt battery, theseries-connection of capacitors usually only takes up part of thevoltage in the cable. The chains of series-connected capacitors includedin the capacitor bank for series compensation are also arranged inseries with the cable to be compensated.

[0007] A conventional capacitor bank comprises a plurality ofcapacitors. Such a capacitor in turn comprises a plurality of capacitorelements in the form of capacitor rolls. The capacitor rolls areflattened and stacked one on top of the other to form a stack 1 m tall,for instance. A very large number of dielectric films with intermediatemetal layers will be arranged in parallel in the vertical direction ofthe stack. When a voltage applied over the stack increases, the stackwill be compressed somewhat in vertical direction, due to Coulomb forcesthat act between the metal layers. For the same reason, if the voltagedecreases the stack will expand somewhat in vertical direction. Thestack formed has a specific mechanical resonance frequency or naturalfrequency, which is relatively low. The mechanical resonance frequencyof the stack is amplified by specific frequencies of the current, whichmay produce a loud noise. The mains frequency constitutes such afrequency. However, amplification of the mechanical resonance frequencycan also be effected by harmonics in the current.

[0008] An example of a power capacitor of this known type is describedin U.S. Pat. No. 5,475,272. A high-voltage capacitor constructed from aplurality of capacitor elements stacked one on top of the other andplaced in a common container, is thus described here. The container ismade of metal in conventional manner. The electrical lead-throughs aremade of porcelain or polymer. The publication also describes variousalternative couplings for connecting the capacitor elements in series orin parallel.

DESCRIPTION OF THE INVENTION

[0009] In known capacitors of this type the capacitor elements areimpregnated with oil. The oil is also arranged to surround the capacitorelements and thus fill up the space between these and the wall of thecontainer. Oil is satisfactory from the insulation aspect but entails anumber of drawbacks. Damage to the container or defective sealing mayresult in oil leakage which may damage the function of the capacitor aswell as contaminating the environment.

[0010] Against this background, the object of the present invention isto overcome the problem of oil leakage from a power capacitor of thetype under consideration.

[0011] From a first aspect of the invention this object is achieved by apower capacitor of the type described in the preamble to claim 1comprising the characteristic features defined in the characterizingpart of the claim. The insulating medium in the form of a dielectricfluid, e.g. an oil comprising a gelling component. The dielectric fluidmay be electrically insulating oil to which gelling components have beenadded. In this context it should be understood that the component mayconsist of a mixture of part-components. The gel surrounding thecapacitor elements in the container thus replaces the oil normally usedfor this purpose. Any damage to the container will not therefore resultin oil leakage since no liquid oil is present. The consistency of thedielectric fluid prevents the formation of drops and it is thereforeunable to leak out. Since the container is made of a polymer materialand therefore yields to a certain extent and is negligibly sensitive tocracking, it has properties of significance in combination with theenclosed gel. The material combines good insulation ability with otherdesired features such as strength, manageability and cost. A design inaccordance with the invention also offers favourable conditions forovercoming the problem of thermal conduction and insulation around theedges of the capacitor windings, which is a particular problem withpower capacitors for high voltage.

[0012] It is known per se to gel an oil for use in electricalarrangements. PCT/SE 98/02314, for instance, describes the arrangementof an electrical arrangement comprising an electric conductor and aninsulation system with a porous, fibre-based or laminated structure. Thestructure is impregnated with a dielectric fluid that is caused tosolidify to a gel. The publication describes, inter alia, an applicationfor impregnating a capacitor bank wound from metal and plastic foil.However, a capacitor element impregnated in this way does not eliminatethe problem of leakage from the oil surrounding the capacitor elementsin a container. This is because said arrangement describes a gel systemin which the oil is thermo-reversible, i.e. at high temperature itbecomes fluid. Neither does the publication solve this type of problem.

[0013] Additional examples are described in JP 716 12 68 and JP 103 26721. However, this does not deal with power capacitors for high voltageeither. JP 103 26 721 shows a capacitor in which the gel is intended tosuppress mechanical vibrations. The object is thus completely differentfrom that of the present invention, which is focused on the task ofavoid an insulating fluid leaking out through the container. JP 103 26721 shows a capacitor in which one side wall consists of urethane resin.The purpose is to prevent electrically conducting material penetratingout if the capacitor breaks, by avoiding cracks in the material throughthe addition of a more flexible material in the form of a gel. Here,too, it is a question of the gel being intended to achieve mechanicalsuppression.

[0014] In a preferred embodiment of the power capacitor in accordancewith the invention the gel state of the dielectric fluid is thermostablethroughout the entire temperature range occurring when the capacitor isin operation. Increased security against the occurrence of oil leakageis obtained by choosing the gelling component so that the gel state isretained even at relatively high temperatures. In accordance with apreferred embodiment the dielectric fluid is silicon-based, thisapplying in particular to the gelling silicon component. A capacitor isthus achieved which is extremely advantageous from the environmentalaspect, for instance. A gel system that instead contains components suchas polyurethane and/or isocyanates does not have such environmentaladvantages. Since these produce toxic gases in the event of a fire, theycontribute to a hazardous working environment during manufacture anddemands for safe waste management and destruction. Toxic gases areproduced in the event of fire in a capacitor containing oil inaccordance with said PCT/SE 98/02314. Furthermore, a gel system withsuch components has the drawback that these swell greatly and negativelyinfluence the metallising film. Since an embodiment with metallisedfilm, i.e. metal-coated film, is advantageous, this is a considerabledrawback. Tests have shown that the films may even be destroyed. Thesedrawbacks are avoided with a silicon-based gel system. This is thereforean embodiment of great significance.

[0015] The present invention is particularly advantageous forapplication in a power capacitor which is produced in known manner fromcapacitor elements in the form of rolled film of plastic and metal or ametal-coated film, wherein the gel is arranged to impregnate the woundcapacitor element, possibly at its end portions, in order to avoidpartial discharges. This thus constitutes a preferred embodiment of thepower capacitor in accordance with the invention. Alternatively, such awinding can be performed dry.

[0016] In an alternative embodiment of such a power capacitor, a seconddielectric fluid is arranged in the space between turns of the winding,which second dielectric fluid is in liquid form, i.e. not gelated.

[0017] The gel surrounding the capacitor elements in the containershould fulfil certain requirements. It should thus display high shearingstrength in gelated state, good thermal conductivity, high electricstrength, be sufficiently electrically insulating and be thermostablewithin the temperature range occurring during operation.

[0018] In accordance with a preferred embodiment the dielectric fluidcomprises an electrically insulating oil. The fluid is thus of a typethat in high degree is capable of fulfilling said requirements. Fromthis aspect, it is particularly suitable for the oil to comprise siliconoil.

[0019] In accordance with a preferred embodiment of the invention thegelling component comprises silicon, preferably polydimethyl siloxanewith at least some vinyl substitutes, i.e. vinyl side groups.

[0020] In accordance with another preferred embodiment, the gellingcomponent comprises silane-functional cross-linking agent. In apreferred alternative this cross-linking agent comprises silicon,suitably polydimethyl siloxane, with at least some silane substitutes.

[0021] The quantity of silane-functional cross-linking agent ispreferably 1-80 per cent by weight.

[0022] The preferred gelling component, the preferred alternativethereof and the preferred content thereof contribute to the dielectricfluid acquiring favourable properties as regards the above requirements.

[0023] In a particularly preferred embodiment the dielectric fluid alsocomprises metal complex, which further contributes to satisfying theabove requirements. Here, too, the quantity of metal complex mixed in is2-4000 ppm, preferably 10-2000 ppm, which has been found to constitute asuitable amount.

[0024] In accordance with yet another preferred embodiment thedielectric fluid comprises silicon liquid of low molecular weight,preferably polydimethyl siloxane liquid. In this case also, a fluid isobtained that in gelled state satisfactorily fulfils the requirementsset.

[0025] In accordance with another embodiment of the invention thedielectric fluid comprises an agent that retards gelation. This permitsa well controlled and extended gelling process that facilitatesmanufacture and contributes to achieving good quality of the gelfunction.

[0026] A suitable quantity of the gelation-retarding component is0.001-4 per cent by weight. In accordance with another preferredembodiment the composition of the dielectric fluid is 1-80 per cent byweight, preferably 20-50 per cent by weight, silane-functionalcross-linking agent, 2-4000 ppm, preferably 10-2000 ppm metal complex,0-60 per cent by weight, preferably 10-50 per cent by weightpolydimethyl siloxane of low molecular weight, 0-4 per cent by weightgelation-retarding agent and the remainder polydimethyl siloxane withvinyl substitutes.

[0027] With such a composition the fluid acquires very suitableproperties for insulating medium that fulfils the necessaryrequirements.

[0028] In accordance with an alternative, also preferred, embodiment thedielectric fluid comprises a vegetable oil, possibly mixed with siliconoil.

[0029] In accordance with a further preferred embodiment the gellingcomponent comprises a vegetable oil.

[0030] In accordance with yet another embodiment of the invention atnormal operating temperature the pressure in the gel is at leastequivalent to atmospheric pressure.

[0031] In accordance with a preferred embodiment each capacitor elementis substantially circular-cylindrical in shape and the inside of thecontainer has corresponding circular-cylindrical shape so that thecontainer closely surrounds each capacitor element, the axial directionof each capacitor element being oriented to coincide with the axialdirection of the container.

[0032] Since the inside of the container has a circular-cylindricalshape corresponding to the cylindrical shape of the capacitor elementsso that the container closely surrounds the capacitor elements, acapacitor is obtained that is as compact as possible and suited to anadvantageous and electrically favourable shape of the elements from amanufacturing point of view.

[0033] In accordance with another embodiment the container is made of anelectrically conducting material. The insulation between the capacitorelements and the container can therefore be simpler without risk ofdischarge between capacitor elements and container. Furthermore, theelectrical connections of the capacitor can be made extremely simple andthe creepage distance necessary between them can be provided by thecontainer itself. With the simplification of the insulation andelimination of the lead-throughs, the capacitor will also be relativelycompact, thereby enabling compact capacitor banks to be built.

[0034] A second aspect of the invention relates to the use of a gelleddielectric fluid to insulate capacitor elements arranged in a container.In preferred embodiments the gel has a composition corresponding to thatstated above for the power capacitor in accordance with the invention.Similar advantages as those described above with regard to the inventedpower capacitor are gained with the use in accordance with theinvention.

[0035] From a third aspect the object is achieved by means of the methoddefined in claim 23. A power capacitor in accordance with the inventionis obtained in a practical way by means of this method

[0036] In accordance with a preferred embodiment of the method inaccordance with the invention the dielectric fluid is degassed beforebeing introduced into the container. This increases the functionalreliability of the capacitor since air bubbles are avoided in the gel,which might cause the appearance of surface glow. Such surface glow cancause erosion in the long run.

[0037] The above and other preferred embodiments of the power capacitorin accordance with the invention, the invented use and the inventedmethod are defined in the sub-claims to respective claims 1, 21 and 23.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic view in perspective of a capacitor inaccordance with a first embodiment of the invention,

[0039]FIG. 2 illustrates a detail from FIG. 1,

[0040]FIG. 3 constitutes a graph illustrating the heat development inthe capacitor element shown in FIG. 2,

[0041]FIG. 4 is an enlarged radial part section through the detail inFIG. 2,

[0042]FIG. 4a is a section corresponding to FIG. 4, but illustrating analternative embodiment,

[0043]FIG. 4b is a section corresponding to FIG. 4, but illustratinganother alternative embodiment,

[0044]FIG. 5 is a longitudinal section through a capacitor element inaccordance with an alternative embodiment,

[0045]FIG. 6 shows two capacitor elements as shown in FIG. 5, connectedtogether,

[0046]FIG. 7 is a view in perspective of a capacitor in accordance withanother embodiment of the invention.

ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

[0047]FIG. 1 shows schematically the design of a capacitor in accordancewith the invention. It consists of an outer container 1 of polyethylenewhich encloses, in this case, four capacitor elements (2 a-2 d). Thecontainer 1, like the capacitor elements 2 a-2 d, iscircular-cylindrical. The capacitor elements 2 a-2 d are connected inseries. Connection terminals 3, 4 are arranged at each end of thecapacitor. Each terminal consists of a conducting foil mounted in thematerial of the container and extending therethrough. A gel 10 isarranged between the capacitor elements 2 a-2 d and the container. Thegel serves as electrical insulation and thermal conductor.

[0048]FIG. 2 shows an individual capacitor element comprisingmetal-coated polymer films tightly rolled to a roll. The capacitorelement 2 has an axially running hole 6 running centrally through it,which may be used for cooling the element. Typical dimensions for such acapacitor element are a diameter of 100-300 mm, a hole diameter of 20-90mm, preferably at least 30 mm, and a height of 50-800 mm. Such acapacitor element is intended for a voltage of about 1-15 kV. Acapacitor element with a diameter of 200 mm, a hole diameter of 60 mmand a height of 150 mm, for instance, is intended for a voltage of about4-10 kV. Up to 40 kV is thus obtained with four of these connected inseries, as shown in FIG. 1, and 80 kV is obtained with eight capacitorelements, etc.

[0049] Thermal losses arise in the capacitor element 2, resulting ininternal heating of the element. The maximum temperature is critical forthe electrical dimensioning. Higher temperature forces lower stress,which leads to lower output per volume unit, i.e. it has considerableinfluence on the consumption of material and the cost. In a cylindricalvolume with homogenous heat generation, and with no opening at thecentre, the temperature profile in radial direction will acquire anasymptotic appearance as indicated by the broken curve in FIG. 3. If thecapacitor element is provided with a central opening 6 with radius Ri,the temperature profile will follow the unbroken curve in FIG. 3. Forcedcooling is also possible if necessary. The temperature profile obtainedwill then be as indicated by the dotted line in FIG. 3.

[0050]FIG. 4 shows an enlarged radial part section through the capacitorelement in FIG. 2. The part section shows two adjacent turns of themetal-coated film. The film 8 a and 8 b, respectively, is approximately10 μm in thickness and the material is polypropylene. The metal layer 9a, 9 b is approximately 10 nm thick and consists of aluminium or zinc ora mixture thereof, which has been vaporised onto the polypropylene filmprior to rolling. With such a metallised film an electric stress E inthe order of 250 V/pm can be reached. The technique of manufacturing acapacitor element in this way is already known and a more detaileddescription is therefore superfluous. Alternatively the capacitorelements can be built up using film foil technology where propylene filmand aluminium foil are rolled up together. However, the use ofmetallised film has the advantage of self-healing and allows higherelectrical stress and higher energy density than with the film foiltechnology.

[0051] The metal layer covers the plastic film from one side edge up toa short distance from its other side edge. A random area 16 a of thefilm 8 a thus lacks metal-coating. In similar manner a random area 16 bof the film 8 b lacks metal coating. The exposed random area 16 b of thefilm 8 b, however, is at the opposite end edge from that on the film 8a. Electrical connection for the layer 9 a is obtained at the upper endof the element, seen in the drawing, and at the lower end of the layer 9b so that a plus electrode is obtained in one direction and a minuselectrode in the other. To ensure efficient electrical contact the endportions may be sprayed with zinc.

[0052] In the modified embodiment shown in FIG. 4a the capacitor elementhas inner series-connection. Here the metal layer 9 a, 9 b on eachplastic film 8 a, 8 b is divided into two portions 9 a′, 9 a″ and 9 b′,9 b″, respectively, separated by an uncoated part 17 a, 17 b,respectively. It is also possible to divide the metal layers into morethan two portions. Each pair of metal-layer portions, e.g. 9 a′ and 9b′, forms a part capacitor element which is connected in series.

[0053]FIG. 4b shows a variant of the modified embodiment, where themetal layer 9 a on only the one plastic film 8 a is divided into twoparts 9 a′, 9 a″ separated by an uncoated part 17 a, whereas the metallayer 9 b on the other plastic film 8 b is undivided. Each of the parts9 a′ and 9 a″ extends right out to the edge of the film 8 b so that theelectrical connection in this case occurs to the same film 8 b. Themetal layer 9 b on the other plastic film terminates on both sides ashort distance 16 a, 16 b from the edge of the film and is thus notelectrically connected in any direction.

[0054] The gel between the capacitors elements (2 a-2 d) and thecontainer consists of a component sold under the trade name Silgel®612from Wacher-Chemie GmbH and comprises gel-forming components.Low-viscous silicon oil is mixed into this component. A component soldunder the trade name “Inhibitor PT 88”, also from Wacher-Chemie GmbH,constituting a gelation-retarding component, is mixed in in analternative embodiment. A suitable silicon oil may be an oil sold underthe trade name “Dow Corning®Silicone Transformer Liquid” from DowCorning.

[0055] By way of example the liquid mixture that is to form the gel maybe composed of about 60-70% of the gelling component, half consisting ofSilgel®612A and half of Silgel®612B. The basic component, i.e. thesilicon oil constitutes about 30-40%, the lower proportion beingapplicable if an inhibitor is used. The remainder, i.e. up to a few percent, consists of the gelation-retarding component.

[0056] Experiment has shown that at a treating temperature of 23° C.solidification occurs in about an hour if no gelation-retardingcomponent is present. With a mixture of 0.5 per cent by weight“Inhibitor PT 88”, the solidification time is extended to just over 10hours. With 1% a solidification time of about 100 hours is achieved, andwith more than 2% the time will be over 150 hours. The inclusion ofabout 1% gelation-retarding component is probably suitable and givessufficiently long solidification time at 60° C. treating temperature.

[0057] The liquid mixture is permitted to penetrate between the filmlayers so that the capacitor element becomes impregnated at least at theside edges. The liquid with the various components is degassed andcombined to a mixture. The mixture is introduced through an inlet in thecontainer by means of a pressure difference achieved by means of a pumpor a vacuum, for instance.

[0058]FIG. 2 illustrates how a power capacitor in accordance with thepresent invention can be constructed for various types of capacitorelements. In all cases a capacitor element 2 is surrounded in acontainer 1 by the dielectric fluid 10 comprising gelling component, andis in gel form in the container.

[0059] In principle, the capacitor element 2 may be constructed inaccordance with three different alternatives as regards the present ofdielectric fluid inside the element. In accordance with a firstalternative the capacitor element 2 may be dry, i.e. no dielectric fluidat all is present inside its winding. According to a second alternativethe capacitor element contains a dielectric fluid that is gelled inequivalent manner to the surrounding gel 10. This may be particularlyrelevant in the end regions A. According to a third alternative thecapacitor element 2 is impregnated with a dielectric fluid such as anoil which does not gel. Here, too, it may be a question of only the endregions A being impregnated.

[0060] The first alternative is primarily of interest in the case oftightly wound capacitor elements, particularly of the type having ametal-coated plastic film. The other two alternatives are primarily ofinterest for loosely wound capacitor elements, particularly of the typein which separate plastic films and metal foils are used in the winding.

[0061]FIG. 5 shows a longitudinal section of an alternative embodimentof a capacitor element 2′ in accordance with the invention. Thecapacitor element is divided into three sub-elements 201, 202, 203 whichare concentric with the common axis designated A. The outermostsub-element 201 is substantially tubular, with an inner side 204surrounding the intermediate sub-element 202 with a small space. Theintermediate sub-element similarly has an inner side 205 that closelysurrounds the innermost sub-element 203. The innermost sub-element 203has a central channel 206 running through it. The three sub-elementshave different radial thickness, the outermost having the smallestthickness. The sub-elements thus have substantially the samecapacitance. Insulation 207 is arranged between the sub-elements.

[0062] The sub-elements are connected in series. Two radially adjacentsub-elements have one of their coupling points at the same end. Theoutermost sub-element 201 is thus connected by the coupling member 210to the intermediate sub-element 202 at one end of the capacitor element2′, and the intermediate sub-element 202 is connected by the couplingmember 211 to the innermost sub-element 203 at the other end of thecapacitor element 2′. This means that the connections 212, 213 for thecapacitor element 2′ are located at opposite ends thereof.

[0063] If the number of sub-elements is greater than three, e.g. five orseven, connection of the coupling points at the ends of the sub-elementsis continued alternately in the same way.

[0064]FIG. 6 illustrates how a plurality of capacitor elements of thetype shown in FIG. 5 can be connected together in series. The figureshows two such capacitor elements 2′a, 2′b. The connection 212 of thelower capacitor element 2′b at the upper end of the inner sub-element203 is coupled to the connection 213 of the upper capacitor element 2′aat the lower end of the outermost sub-element 201. Insulation 214 isarranged between the capacitor elements in order to deal with thepotential differences occurring in this type of capacitor element.

[0065]FIG. 7 shows another example of a power capacitor in accordancewith the invention. In this example the design of the container 301 andcapacitor element 302 is of conventional type. The capacitor container301 is thus box-shaped and the capacitor element 302 is wound toflattened units stacked one on top of the other. The electricalconnection terminals 303, 304 are directed the same way. A gel 310 isarranged in the space between the capacitor elements 302 and container301 in similar manner to the embodiments described above.

1. A power capacitor for high voltage comprising at least one capacitorelement (2 a-2 d) enclosed in a container (1) in such a way that a spaceis formed between each capacitor element (2 a-2 d) and the container(1), characterized in that the capacitor also comprises an insulatingmedium arranged inside the container (1) in the form of a dielectricfluid (10) comprising gelling component, which insulating mediumsubstantially fills said space, and in that the container (1) is made ofa polymer material, e.g. polyethylene.
 2. A power capacitor as claimedin claim 1, characterized in that the gelling component comprisessilicon.
 3. A power capacitor as claimed in claim 2, characterized inthat the silicon comprises polydimethyl siloxane.
 4. A power capacitoras claimed in claim 2 or claim 3, characterized in that the siliconcomprises vinyl substitutes.
 5. A power capacitor as claimed in any oneof claims 1-4, characterized in that the gel state of the dielectricfluid (10) is thermostable throughout the entire temperature rangeoccurring when the capacitor is in operation.
 6. A power capacitor asclaimed in any one of claims 1-5, characterized in that each capacitorelement (2 a-2 d) comprises film that is rolled several turns to form aroll, which film comprises layers (8) of plastic and layers (9) of metalor metal-coated plastic film, a second dielectric fluid being arrangedin the space between turns of the winding, at least in the region of theends of the roll, which second dielectric fluid is in liquid form.
 7. Apower capacitor as claimed in any one of claims 1-5, characterized inthat each capacitor element (2 a-2 d) comprises film that is rolledseveral turns to form a roll, which film comprises layers (8) of plasticand layers (9) of metal or metal-coated plastic film, the dielectricfield (10) being also arranged in the space between turns of thewinding, at least in the region of the ends of the roll.
 8. A powercapacitor as claimed in any one of claims 1-7, characterized in that thedielectric fluid (10) comprises an electrically insulating oil,preferably silicon oil.
 9. A power capacitor as claimed in claim 2 orclaim 3, characterized in that the gelling component comprisessilane-functional cross-linking agent.
 10. A power capacitor as claimedin claim 9, characterized in that the silane-functional cross-linkingagent comprises silicon, preferably polydimethyl siloxane, with at leastsome silane substitutes.
 11. A power capacitor as claimed in claim 7 orclaim 8, characterized in that the quantity of silane-functionalcross-linking agent is 1-80 per cent by weight.
 12. A power capacitor asclaimed in any one of claims 2-11, characterized in that the dielectricfluid (10) also comprises metal complex.
 13. A power capacitor asclaimed in claim 11, characterized in that the quantity of metal complexis 2-4000 ppm, preferably 10-2000 ppm.
 14. A power capacitor as claimedin any one of claims 2-13, characterized in that the dielectric fluid(10) comprises silicon liquid of low molecular weight, preferablypolydimethyl siloxane liquid.
 15. A power capacitor as claimed in anyone of claims 2-14, characterized in that the dielectric fluid (10)comprises an agent that retards gelation.
 16. A power capacitor asclaimed in any one of claims 2-15, characterized in that the compositionof the dielectric fluid (10) is 1-80 per cent by weightsilane-functional cross-linking agent, 2-4000 ppm, preferably 10-2000ppm metal complex, 0-60 per cent by weight, preferably 10-50 per cent byweight polydimethyl siloxane of low molecular weight, 0-4 per cent byweight gelation-retarding agent and the remainder polydimethyl siloxanewith vinyl substitutes.
 17. A power capacitor as claimed in claim 8,characterized in that the dielectric fluid (10) comprises a vegetableoil.
 18. A power capacitor as claimed in any one claims 1-17,characterized in that the dielectric fluid (10) is under a pressure atleast equivalent to atmospheric pressure.
 19. A power capacitor asclaimed in any one of claims 1-18, characterized in that each capacitorelement is substantially circular-cylindrical in shape and the inside ofthe container (1) has corresponding circular-cylindrical shape so thatthe container closely surrounds each capacitor element, that axialdirection of each capacitor element being oriented to coincide with theaxial direction of the container.
 20. A power capacitor as claimed inany one of claims 1-19, characterized in that the container (1) enclosesa plurality of capacitor elements (2 a-2 d) connected in series, and inthat an electric connection terminal (3,4) is arranged at each end ofthe container (1), the container (1) itself constituting insulationbetween the connection terminals.
 21. The use of a gelled dielectricfluid as insulating medium for insulating one or more capacitor elements(2 a-2 d) in a power capacitor for high voltage, which capacitorelements are arranged in a container (1) of a polymer material, e.g.polyethylene.
 22. Use as claimed in claim 21, characterized in that thedielectric fluid (10) is of a type defined in any one of claims 2-17.23. A method for manufacture of a power capacitor as claimed in any oneof claims 1-20, characterized in that at least one capacitor element (2a-2 d) is arranged in a container, in that a dielectric fluid capable ofgelling is introduced into the space between each capacitor element (2a-2 d) and the container (1), after which the dielectric fluid (10) iscaused to gel.
 24. A method as claimed in claim 23, characterized inthat the dielectric fluid is degassed before being introduced into thecontainer.